Well, I've had a long time to think about this OP contact issue. It is in all the literature that they must cut contact. But, if I were to suggest that, I would make myself a target. If C was, she would be. I believe this "rule" of no contact has to be taken more as a guideline. I believe there are few rules.
Ther problem is that OM is part of her circle of friends. This circle has many positive things going for it, and is large. As you point out, she's no longer so fast and tight with the separated and divorced contingent, but much more involved these days with the married members. She doesn't hang out with the SSW any more, and she's not likely to accept invitiations to events that would preclude family participation. Overall, the circle is a good influence, and the bad influences are waning.
The point is that, Yes, I agree, contact with OM being eliminated would be good. Contact with the rest of the circle is also good. I see a trend toward her gravitating toward the positive influences and away from the negatives. I believe her that she has little direct contact with him. What is truly most important here is that she know that it is the consequences of her choices she must deal with. Her choices. If an outside influence (me, or the C) "forces" her to choose a certain way, the impact will be less, and maybe even negative. It will create a situation against which she may rebel, or blame for things not working (If I had done it MY way, I would be happier....). If she makes the choices, and things don't work out, she has no one to blame, and must take responsibility for her own happiness.
What this requires on my part is almost unbearable patience and detachment from the drama. I am getting better at it, and I'm feeling confident.
Now you point out that the C appears to be leading us in a direction. I have not missed that. I see her leading W more and more. I would not be surprised if at some point C does point out that OM contact is not good, but I believe she can see the futility of it right now. We are slowly getting to a point where it's safe for W to acknowledge this contact, to make it less and less secretive. As it becomes more open, it will become more OK for me to express that I would prefer there was less of it. If I rush that, she will just as likely go back "underground" with it.
That is why my latest "speach" was about how it's none of my business. I truly believe that. It's none of my business. However, that doesn't mean that I'm not entitled to an opinion, to a preference. But, if I had expressed that opinion and preference at the same it I acknowledged it's none of my business, what would have been heard was my protests. My acknowledgement that it was none of my business would have been viewed as "conditional", and the impact, the release of control and the granting of a safe space would have been diminished if not negated.
This is all part of the required patience, in my opinion.
